Suspension polymerization is widely used industrially as a method for preparing common thermoplastic resin such as vinyl chloride resin, styrene resin, acrylic resin and methacrylic resin, from the viewpoints that controlling polymerization heat is easy and polymer particles can easily be recovered by simply conducting dehydration and drying.
However, when polymer particles are prepared by suspension polymerization, the particle size thereof usually exhibits a broad distribution and fine particle polymers, which are extremely far from the desired particle size, are also produced at the same time. As a result, unwanted results occur, such as filtration properties become extremely poor due to clogging of the filter fabric caused by the fine powder in the dehydrating step, fine powder flows out into the wastewater from dehydration, problems in the steps often occur due to clogging of fine powder, operation environment becomes poor due to generation of dust and risk of dust explosion increases.
The reason that fine powder develops in suspension polymerization is because considerable emulsification is conducted when beginning suspension polymerization as the monomer used in polymerization has some water-solubility or because the monomer drops are over-dispersed, as the monomer is subjected to strong shearing by stirring. Until now, in order to overcome such disadvantages, studies have been conducted such as selecting the type and amount of dispersion stabilizer added when polymerizing regarding the former (JP-A-49-88987) and homogenizing the fluid state by improving the stirring blade regarding the latter. However, even when these methods are used together, completely preventing development of fine particles is difficult in many cases, becoming a large disadvantage to manufacturers with respect to wastewater treatment.
Also, in emulsion polymerization, when granulated and recovered graft copolymer resin is melted and kneaded with rigid plastic, impact strength is exhibited because the graft copolymer is redispersed within the rigid plastic while maintaining the particle size at polymerization. Therefore, emulsion polymerization is widely used as a process for preparing an impact modifier of rigid plastic such as vinyl resin, styrene resin, acrylonitrile-styrene resin, polycarbonate resin and polyether resin.
Usually, in order to recover the desired graft copolymer from an emulsion polymer latex, a granulation step of coagulating and recovering the latex is necessary. The granulation step has a large influence not only on the particle size distribution of the recovered particles, the amount of fine powder and powder properties such as fluidity, but also on productivity of after-treatment such as dehydrating properties and drying properties. Conventionally, in the case of recovering particle polymers from a latex prepared by emulsion polymerization, usually a coagulating agent is added to the latex, a solid-liquid dispersion is formed by the steps of coagulation in the liquid phase and thermal treatment and thereafter, dehydration and drying are conducted to obtain powdery synthetic resin. However, in the case of this method, the shape of the powder is indefinite and problems often occur. For example, problems in the steps often occur, as a considerable amount of fine powder is included, and operation environment becomes poor due to generation of dust. As a result, various improvements have been considered by suggestions of new granulation methods including vapor-phase coagulation (JP-A-52-68285) and delayed coagulation (JP-A-60-217224). However, in spite of such efforts, the amount of water and electric power used in the coagulation step, washing step and drying step of the various granulation methods described above is significantly larger than in suspension polymerization process. From the viewpoint of energy conservation, the above methods are still not satisfactory and development of a new granulation method including designing resin is desired.
On the other hand, in the field of mold processing, a polymer which is usually used as thermoplastic resin, that is a polymer obtained by suspension polymerization which is processed into an article by a molding machine, is hardly processed alone and usually a quality modifying agent such as an impact modifier or processability modifying aid. As mentioned above, such quality modifying agents are usually prepared by emulsion polymerization and recovered as powder. Usually, in order to obtain a molded article with excellent properties, processors conduct the so-called compounding process of adding and mixing a quality-modifying agent to the above polymer and thereafter conducting mold processing. However, the present conditions are that this compounding process is forcing decrease in operational efficiency to processors, as operation environment becomes poor due to generation of dust.
Also, as a toner for developing static-charged images which has excellent frictional charge properties and excellent moisture-absorption properties, JP-A-57-45558 discloses a toner for developing static-charged images comprising particles formed by suspension polymerization and a coating layer comprising fine particles formed by emulsion polymerization on the surface of the particles. However, in this method, after recovering the polymer obtained by suspension polymerization once, the coating step with the polymer obtained by emulsion polymerization is conducted and therefore, this method does not solve the problems of fine powder caused by suspension polymerization.
As a toner in which both adhesion and low-temperature fixation of the toner are achieved, JP-A-2000-112174 discloses a toner for developing static-charged images, in which at least 95% of the surface of the polymer particles obtained by suspension polymerization is coated with a polymer obtained by emulsion polymerization. However, in this method, the polymer solid content concentration of the polymer suspension, when coating the polymer obtained by suspension polymerization with the polymer obtained by emulsion polymerization, is at most 25% and in such conditions, the effect of improving filtration properties cannot be obtained. Also, the average particle size of the polymer obtained by suspension polymerization which is used is 2 to 10 μm, therefore being fine powder itself. Consequently, this method is not a method which solves the problems of fine powder caused by suspension polymerization of the present invention.
U.S. Pat. Nos. 4,307,134, 4,333,969, 4,333,970 and 4,385,156 disclose a method of coating a styrene polymer obtained by suspension polymerization with a polymer obtained by emulsion polymerization for a polymer used in foamed molded articles. However, these methods are not methods which solve the problems of fine powder caused by suspension polymerization.
JP-A-6-179754 discloses a suspension polymer, which contains non-adhesive granulated powder and has a glass transition temperature lower than 50° C., wherein the granulated powder has a non-adhesive coating comprising an emulsion polymer having a glass transition temperature higher than 50° C., and a process for preparing the same. This method is a method which can be used to prevent blocking of the polymer obtained by suspension polymerization which has a low glass transition temperature. Furthermore, a coating with an excess amount of the polymer obtained by emulsion polymerization causes development of fine particles when dehydrating and therefore, the polymer obtained by emulsion polymerization is 2 to 10 parts based on 100 parts of the polymer obtained by suspension polymerization. Consequently, this method is not a method which solves the problems of fine powder caused by suspension polymerization of the present invention.
Furthermore, JP-A-56-50907 discloses the method of partially coagulating an emulsion polymer latex, adding thereto an ethylenic monomer while stirring, converting the polymerization system from an emulsion system to a suspension system and conducting suspension polymerization, as an emulsion-suspension polymerization method. According to this method, a polymer obtained by suspension polymerization which is usually used as thermoplastic resin, and polymer particles obtained by emulsion polymerization, which are an impact modifier, are unified to prepare complex particles. Additionally, when this method is used, the coagulating (granulating) step which is essential for recovering the emulsion polymer latex can be omitted, the obtained particles have an excellent spherical shape and the amount of fine powder is extremely little. Also, granulated particles having low dry load (low water content after dehydration) are obtained and so the amount of energy consumption is more advantageous than in the current emulsion polymerization process. Consequently, the problems of suspension polymerization and emulsion polymerization process can be significantly reduced. However, this method is inferior with respect to productivity, in that increase in viscosity of the system when converting from an emulsion system to a suspension system is remarkable and the total polymerization time to continue conducting suspension polymerization after developing the polymerization scale or completing emulsion polymerization is extremely long.
Therefore, a method which can solve all at once the problem of fine powder caused by suspension polymerization process, the problem of reducing energy consumption when drying caused by emulsion polymerization process and the problem of the compounding process for processors is currently not available.